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[GDN] Optimize b_dg computation in chunk_bwd_kernel_dqkwg#USE_G #823

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[GDN] Optimize b_dg computation in chunk_bwd_kernel_dqkwg#USE_G #823

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[GDN] Optimize `b_dg` computation in chunk_bwd_kernel_dqkwg#USE_G
MzeroMiko Apr 11, 2026
71a2bcc
Merge branch 'main' into main
yzhangcs Apr 18, 2026
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11 changes: 5 additions & 6 deletions fla/ops/common/chunk_o.py
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Original file line number Diff line number Diff line change
Expand Up @@ -275,7 +275,6 @@ def chunk_bwd_kernel_dqkwg(
m_t = o_t < T
m_A = (o_t[:, None] >= o_t[None, :]) & (m_t[:, None] & m_t)
if USE_G:
b_dg = tl.zeros([BT], dtype=tl.float32)
g += bos * HV + i_h
dg += bos * HV + i_h
p_g = tl.make_block_ptr(g, (T,), (HV,), (i_t * BT,), (BT,), (0,))
Expand All @@ -287,27 +286,27 @@ def chunk_bwd_kernel_dqkwg(
else:
b_dg_last *= exp(b_g_last)
b_dq = b_dq * exp(b_g)[:, None] * scale
b_dg += tl.sum(b_dq * b_q, axis=1)

if USE_EXP2:
b_dk = b_dk * tl.where(m_t, exp2(-b_g + b_g_last), 0)[:, None]
else:
b_dk = b_dk * tl.where(m_t, exp(-b_g + b_g_last), 0)[:, None]
b_dg -= tl.sum(b_k * b_dk, axis=1)
b_dg_last += tl.sum(b_dk * b_k)

if USE_EXP2:
b_ds = tl.where(m_A, b_ds * exp2(b_g[:, None] - b_g[None, :]), 0) * scale
else:
b_ds = tl.where(m_A, b_ds * exp(b_g[:, None] - b_g[None, :]), 0) * scale
b_ds2 = b_ds * tl.dot(b_q, tl.trans(b_k))
b_dg += tl.sum(b_ds2, axis=1)
b_dg -= tl.sum(b_ds2, axis=0)

b_ds = b_ds.to(b_k.dtype)
# [BT, BK]
b_dq += tl.dot(b_ds, b_k)
b_dk += tl.dot(tl.trans(b_ds), b_q)

b_dg = tl.zeros([BT], dtype=tl.float32)
b_dg += tl.sum(b_dq * b_q, axis=1)
b_dg -= tl.sum(b_dk * b_k, axis=1)
Comment on lines +306 to +308
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@gemini-code-assist gemini-code-assist bot Apr 11, 2026

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medium

The initialization of b_dg to zero followed by incremental updates can be simplified into a single expression. This improves code clarity and avoids redundant operations.

Suggested change
b_dg = tl.zeros([BT], dtype=tl.float32)
b_dg += tl.sum(b_dq * b_q, axis=1)
b_dg -= tl.sum(b_dk * b_k, axis=1)
b_dg = tl.sum(b_dq * b_q, axis=1) - tl.sum(b_dk * b_k, axis=1)


p_dg = tl.make_block_ptr(dg, (T,), (HV,), (i_t * BT,), (BT,), (0,))
# (SY 09/21) revcumsum in a separate kernel due to strange triton compiler issue
# b_dg = tl.dot(tl.where(o_t[:, None] <= o_t[None, :], 1., 0.), b_dg, allow_tf32=False) + b_dg_last)
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